This invention concerns a process for regenerating certain palladium catalysts used in the carbonylation of olefins. Carbonylation refers here to the reaction of olefins with carbon monoxide and active-hydrogen-containing compounds selected from the group consisting of an alkanol or water. Suitable alkanols (ROH) include primary and secondary alcohols, glycols and polyols.
Oxychlorination as used throughout this application refers to processes wherein hydrogen chloride plus oxygen or air are used to regenerate a spent catalyst used for carbonylation of olefins.
The preparation of the fatty acids or fatty acid esters using metal carbonyls or carbonyl precursors to catalyze the carbonylation of olefins is old in the literature, originally involving Reppe and his coworkers and contemporaries. Reviews by C.W. Bird [Chem.Rev. 62, 283 (1962)] document this work. Unfortunately, many of these carbonyl or carbonyl-type catalysts have the disadvantages of inherent toxicity, they require stringent reaction conditions which in turn lead to competing side reactions such as olefin isomerization, polymerization and reduction, and they exhibit poor selectivity to the desired linear acid ester.
Recently, more acceptable homogeneous catalyst systems have been developed which offer substantially improved selectivity in converting olefins to primarily linear fatty acids or linear fatty esters, in good yield, under moderate reaction conditions of temperature and pressure.
As is usually the case, after much more extensive usage, certain drawbacks in the catalysts have become more evident. These include difficulty in maintaining high conversions, high selectivities and high yields after recycling the catalyst several times. These problems are due to catalyst degradation as well as catalyst decomposition, mechanical losses and further catalyst decomposition during the separation of the products from the homogeneous catalysts and the inert solvents of the reaction mixture. Thermal instability of the catalyst is particularly troublesome in the recovery and working-up of certain ligand-stabilized homogeneous palladium catalyst reaction mixtures.
In order to avoid or minimize these problems, the use of molten quaternary ammonium, phosphonium and arsonium salts of trihalostannate(II) or trihalogermanate (II) as both solvent and part of the catalytic entity has been disclosed, particularly in the two United States patents of G.W. Parshall, U.S. Pat. Nos. 3,657,368 and 3,565,823, which are known art, as well as in applicant's more recently filed one parent case listed above. In the parent application certain criteria are discussed which are required in order to establish an advantageous catalytic carbonylation process. They are:
1. A simple and efficient means of separating catalysts from the products, PA1 2. The ability to recycle the catalyst without its substantial deactivation or loss of selectivity. This is particularly important since the palladium catalysts are thermally sensitive, and PA1 3. In addition, to minimize capital costs, it is desirable that the process be able to operate at high concentrations of catalyst in the feed stream.
In the cited U.S. Pat. Nos. 3,657,368 and 3,565,823 are also disclosed two illustrative procedures for isolating ester or acid products, such as are produced by olefin carbonylation, in high purity, from palladium catalysts consisting of dispersions of ligand-stabilized palladium(II) halide in quaternary ammonium, phosphonium and arsonium salts of trihalostannate(II) and trihalogermanate(II). Briefly, one procedures involves solvent extraction, the other, vacuum distillation.
Also disclosed in applications Ser. Nos. 581,320 and 581,395 are two illustrative procedures for regenerating the same palladium catalysts after multiple cycling. These two different procedures involve treatment of the spent catalyst with chlorine or mixed mineral acids. Each procedure is a process improvement which provides a useful means of extending the life of the palladium catalyst, and thereby it allows the development of a more efficient carbonylation process.